Exemplary embodiments pertain to the art of electronic engine controls and, more particularly, to a method of operating an electronic engine control that controls fuel delivery to an engine to compensate for engine rotor speed changes.
Engines, particularly those used to power aircraft, rely on metered flow accuracy provided by a fuel control to provide proper performance. Variations resulting from fuel temperature changes, manufacturing tolerances, calibration/acceptance tests tolerances and the like are included in any accuracy analysis. In a typical fuel control algorithm, an electronic engine control (EEC) schedules fuel flow by establishing an opening of a metering valve (MV) having a flow window. The opening is based on signals from an electrical feedback device. A pressure regulating valve (PRV) is operatively coupled to the metering valve (MV) and is designed to establish a relatively constant pressure differential (delta P) across the flow window. The resulting flow from the metering valve is passed to the engine.
One factor that contributes to metered flow accuracy is maintaining the constant pressure differential (delta P) across the MV. The amount of flow over that which is required by the engine for a given operating condition, or bypass flow, passes across bypass flow windows of the PRV. The amount of bypass flow as well as the pressure drop across the bypass flow window has an impact on a delta P of the MV which then impacts the metered flow through the metering valve. Bypass flow is primarily driven by pump speed which is geared to a rotor of the engine, engine burn flow, pump volumetric displacement and fuel control/actuator (if applicable) internal leakages. The pressure drop across the PRV bypass flow window is primarily driven by engine compressor pressure, engine nozzle drops (which is a function of engine burn flow), and fuel control housing/valve pressure drops. For a specific engine fuel flow, the engine has a range of operating rotor speeds which varies with changing environment conditions such as altitude and engine load requirements such as bleed extraction. Metered flow accuracy is also a consideration when the EEC is operating in an open loop mode such as during start up or when the EEC has flow limits during engine acceleration/deceleration transients. These flow limits protect the engine against surge or flameout events yet also limit the ability for the engine to respond to rapid acceleration/deceleration requests.